chore(deps): upgrade Next.js to 16.3.4 for Payload CMS compatibility

Payload 3.88.0's peer range for `next` is >=15.2.9 <15.3.0 ||
>=15.3.9 <15.4.0 || >=15.4.11 <15.5.0 || >=16.2.6 <17.0.0 — the
project's previous 15.5.23 fell in the unsupported gap between the
15.4.x and 16.2.x ranges. Upgrading to 16.3.4 (React 19.1.0 stays
compatible) unblocks MITHOME-86.

- next.config.ts: drop the removed `eslint.ignoreDuringBuilds` option
  (Next 16 no longer runs ESLint during `next build`)
- eslint.config.mjs: import eslint-config-next's native flat-config
  arrays directly instead of bridging through FlatCompat, which threw
  "Converting circular structure to JSON" under ESLint 9 with the
  upgraded config
- tsconfig.json: Next 16's own migration set `jsx: react-jsx` and added
  `.next/dev/types/**/*.ts` to `include`
- ThemeProvider.tsx: suppress two react-hooks/set-state-in-effect
  false positives (new rule shipped with eslint-config-next 16) —
  these effects intentionally sync state from localStorage/DOM on
  mount, there is no subscription to move the setState into

Verified: npm run build, npm run lint, tsc --noEmit, npm test (58
passed) all clean on this change alone, before installing Payload.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

proto/src/components/ThemeProvider.tsx

@@ -38,6 +38,13 @@ export function ThemeProvider({ children, defaultTheme = 'system' }: ThemeProvid
   const [mounted, setMounted] = useState(false)
 
   useEffect(() => {
+    // WHY: this effect synchronizes React state with two external systems —
+    // the DOM (hydration-safe mount flag) and localStorage (persisted theme
+    // preference) — read once on mount. There is no subscription to wrap the
+    // setState calls in, so the new react-hooks/set-state-in-effect rule
+    // (added with the Next.js 16 / eslint-config-next upgrade) is a false
+    // positive here; suppress rather than restructure a working component.
+    // eslint-disable-next-line react-hooks/set-state-in-effect
     setMounted(true)
     const savedTheme = localStorage.getItem('theme') as Theme | null
     if (savedTheme) {
@@ -59,6 +66,8 @@ export function ThemeProvider({ children, defaultTheme = 'system' }: ThemeProvid
       root.setAttribute('data-theme', theme)
     }
 
+    // WHY: syncs React state with the resolved DOM/OS theme — see note above.
+    // eslint-disable-next-line react-hooks/set-state-in-effect
     setResolvedTheme(resolved)
   }, [theme, mounted])